PNNL

Abstract

Interfacial structure evolution and degradation are critical to the electrochemical performance of LiCoO2 (LCO), the most widely studied and used cathode material in lithium ion batteries. To understand such processes requires precise and quantitative measurements. Herein, we use well-defined epitaxial LCO thin films to reveal the interfacial degradation mechanisms. Through our systematical investigations, we find that surface corrosion is significant after forming the surface phase transition layer, and the cathode electrolyte interphase (CEI) has double layer structure, an inorganic inner layer containing CoO, LiF, LiOH/Li2O and LixPFyOz, and an outmost organic layer containing Li2CO3 and carbonaceous components. Furthermore, surface cracks are found pronounced due to mechanical failures and chemical etching. This work demonstrates a model material to realize the precise measurements of LCO interfacial degradations, which deepens our understanding on the interfacial degradation mechanisms.